Gases such as natural gas, refinery gases and synthesis gas have been purified by the utilization of aqueous alkanolamine solutions for the absorption of acid gases such as CO.sub.2, H.sub.2 S, and COS contained in the gas stream. Ordinarily, a 10 percent to 30 percent by weight alkanolamine solution (e.g., a monoethanolamine solution), flowing countercurrently to the gas stream in an absorption column, is used to remove the acid gases. The process is a continuous and cyclic one which can be reversed at higher temperatures by desorbing the acid gases from the alkanolamine solution.
When steel parts or components are used in such a system, they are subject to both general and local corrosive attack. This is a particular problem in reboilers and heat exchangers where the steel is exposed to a hot, protonated alkanolamine solution. A heat transfering metal surface appears to be especially vulnerable. Previous investigations by others have revealed that under certain conditions, corrosive products such as aminoacetic; glycolic, oxalic, and formic acids were formed. The alkanolamine salts of these acids present the possibility of increased attack upon ferrous metals. Furthermore, the carbonate salt of monoethanolamine can be converted to additional products such as N-(2-hydroxyethyl)-ethylenediamine which has been found to increase the corrosiveness of the amine solution towards steel, particularly under heat transfer conditions.
There are various alternatives available in order to decrease corrosion rates, among them (1) the provision of a side-stream reclaimer to remove corrosive degradation products, (2) the employment of more corrosion resistant materials, (3) greater control of the process conditions, and (4) the inclusion of corrosion inhibitors. Various corrosion inhibitors have been formulated, as indicated by the examples given below, to purify a feedstream of acid gas based either on the presence or the absence of hydrogen sulfide in the feedstream.
A corrosion inhibiting composition for use with an aqueous alkanolamine solution to remove acid gases from a feedstream such as natural gas which does not contain any hydrogen sulfide is taught in U.S. Pat. No. 3,808,140. The corrosion inhibitor described in this patent is limited to a combination of vanadium compounds and antimony compounds and will not tolerate the presence of hydrogen sulfide.
A corrosion inhibiting composition for use with an aqueous alkanolamine solution to remove acid gases from a feedstream such as synthesis gas which contains hydrogen sulfide, is taught in U.S. Pat. No. 4,502,979. This patent discloses the synergistic combination of a nitro-substituted aromatic acid, or a nitro substituted acid salt, with a vanadium compound for providing corrosion inhibition in an alkanolamine gas treating system wherein at least a portion of the acid gas is hydrogen sulfide.
It has now been discovered that under certain process conditions the alkanolamine gas treating process will operate more effectively using vanadium compounds in the plus five valence state alone or in combination with other chemicals to inhibit corrosion in an acid gas feedstream with hydrogen sulfide present.